The present invention relates to marine seismic surveying.
Seismic surveys are carried out to investigate the nature of sediment and rock that lie below the surface. These work by sending acoustic signals into the ground and recording the echoes. The pattern and variation in the echoes can be used to assess the strength and stratification of the underlying material. For marine seismic surveys (at sea, on rivers and on lakes), acoustic pulses are generated in the water and the seismic signals travel down through the seabed before the echoes are recorded.
Various acoustic sound sources are used for marine seismic surveys. Sources well known to practitioners include air guns, water guns, explosives, sparkers, boomers and pingers.
A xe2x80x9cboomerxe2x80x9d sound source is often used in marine seismic surveys. The boomer has an electrical coil that is towed through the water by a survey vessel, with the coil protected from the water by a flexible membrane. FIG. 1 depicts the prior art configuration of a survey vessel 10 towing a boomer coil and plate 14 with a heavy duty power cable 13 having thick insulation. Control electronics and large capacitors are situated a xe2x80x9ccontainment boxxe2x80x9d 11 on the survey vessel 10. A receiver 12 is towed independently behind the survey vessel 10, or alternatively may be towed with the boomer.
A prior art boomer coil and plate 14 is schematically depicted in FIG. 2 to include a frame 15 housing coil wires 16 of circular cross-section. The insulating flexible membrane is shown at 18 in the figure.
The capacitors on the survey vessel are in a large bank to provide the necessary power source. Discharging the capacitors that have been energised to a high potential difference, typically 3000 Volts to 5000 Volts, generates the acoustic signal as described below. Controlling the discharge of the capacitors in a safe manner requires heavy-duty equipment and cables 13.
The coil 16 is formed into a circular disk next to a conductive plate 17 of approximately the same diameter. The coil and plate are placed in the frame 15. The insulating flexible membrane 18 protects the coil and plate. When the energy from the capacitors on the survey vessel 10 is discharged through the coil, the plate moves (xe2x80x9ckicksxe2x80x9d) away, in accordance with an electromotive effect. This sharp movement passes through the flexible membrane into the water. At this interface between the membrane and the water, the energy converts to a pressure wave (the seismic signal) that travels down through the water and to the underlying sediments and rock. A typical time series of the acoustic pulse measured in the water is shown in FIG. 6.
In principle, an array of sources could be used to focus the acoustic wave vertically below the instrument. However, the coils of conventional boomers have a diameter typically around 1 m. A simple two-by-two array would at least double the size and quadruple the weight. Since this makes the boomer much more difficult to deploy, multiple coils grouped to form a focused array have never progressed beyond test prototypes.
A surface-towed boomer is smaller and simpler to deploy than a boomer designed to tow at depth. In most applications, a surface-towed system provides adequate information on the sediments and rock beneath the seabed. However, in some commercially important cases, the ability to tow the system at depth is beneficial. Present deep-tow boomers and their cable systems are large and heavy, requiring special lifting equipment.
As the capacitors need to be energised to high potential differences, the capacitors are large and heavy. Also, the high potential differences are a possible safety hazard requiring strong safety containment to ensure they can be used in a safe operating fashion. This heavy steel safety container adds to the weight. To cope with the high current that flows each time the capacitors are discharged, the cable connecting the capacitors to the coil is also heavy, both to enclose broad-gauge conductors that can safely handle the large currents and to include adequate insulation. Energy recovery from the coil to recharge the capacitors has been used, but is relatively inefficient due to inductive losses in the power cable.
It is an object of the present invention to overcome or substantially ameliorate the above disadvantages and/or more generally, to provide an improved configuration of equipment for conducting seismic surveys.
There is disclosed herein a boomer for generating acoustic signals in a marine seismic survey, the boomer comprising:
a frame adapted to be towed by a survey vessel,
a coil mounted to the frame,
a conductive plate adjacent to the coil,
a capacitor mounted to the frame, and
a control electronics system for delivering electric charge stored in the capacitor to the coil.
Preferably, the frame is buoyant so as to float at a water surface.
Alternatively, the frame can be designed for deep towing.
Preferably, the coil comprises wires of square or rectangular cross-section.
Preferably, an insulating flexible membrane is attached to the frame adjacent to the conductive plate.
Preferably there is a plurality of capacitors.
Preferably, the control electronics system includes switches and current and potential controllers to regulate energy supplied by the capacitor to the coil.
Preferably multiple coils are situated within the frame to focus the acoustic energy.
There is further disclosed herein a method of performing a marine seismic survey, the method comprising:
towing the above-disclosed boomer behind a survey vessel that also tows a receiver,
charging the capacitor,
sending a signal to the control electronics system to discharge the capacitor to thereby energise the coil and kick the conductive plate to provide an acoustic pulse, and
receiving a reflected signal with the receiver.
Preferably, both potential of the coil and current passing through the coil are controlled and varied, to optimise and tune the acoustic pulse.
Preferably, part of the energy of the coil is recovered to discharge the capacitor without inductive losses along a long power cable.